The present invention relates to a device for fixing bone sections separated because of a fracture.
More particularly, the invention relates to a plate and screw assembly for osteosynthesis, allowing immobilising one or more fractures in a long bone, such as a femur or a tibia.
Generally the plates have elongate shape ad are available in different models, depending on the shape and size of the bone they are to be applied to.
For instance, patent application WO 97/08999 discloses a plate for fixing fractures in the end section, or head, of a bone. Such a plate has an elongate portion that can be fastened to the bone through a plurality of screws, and an end portion including a barrel that is inserted into the bone to receive a compression screw. The compression screw is previously screwed into the bone; subsequently, after having located the plate with the barrel, the screw is fastened by a traction screw allowing compression of the plate against the bone.
Generally, compression screws are used in the bone head, whereas conventional screws of small size are used for fastening the plate to the bone shaft.
The barrel, which is integral with the plate, allows a rigid connection of the plate and the screw, thereby eliminating any clearance that might arise between the two parts.
Yet, the presence of the fixed barrel makes the plate application complex, since it is necessary to previously drill a bore in the bone, and hence use of special guides and tools is indispensable.
European patent EP 0 649 635 discloses a plate equipped, at its end portion, with two parallel barrels adapted to be inserted into the bone head. Also in this case the plate is fastened to the bone through a plurality of conventional screws, whereas compression screws are introduced in correspondence of the barrels. The presence of two barrels further increases the application difficulty, since two perfectly parallel suitably offset bores are to be drilled in the bone so as to allow a precise introduction of the barrels.
An alternative solution to using barrels is disclosed in PCT patent application WO 97/20313. That document discloses a special system for coupling a screw head and a corresponding opening in the plate. More particularly, the screw has an expansion head that, when expanding, becomes tightly locked in an opening of complementary shape in the plate. Yet such a solution, while allowing a certain rigidity in the screw-plate connection and making operations simpler, does not provide the same structural rigidity as the barrels.